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Drug dealers and bank robbers beware: your voice could give you away. Linguists are out and about identifying criminals from various voice 'footprints' they leave behind on recordings.

According to the current issue of the ANU Reporter, there is an increasing demand for experts in forensic phonetics as the proliferation of voice-recording technologies means more criminals are being caught on tape.

Dr Phil Rose, a senior lecturer in the Linguistics Program in the Faculty of Arts, said that while linguistic evidence was not comparable to DNA evidence or fingerprinting, it could be an essential plank in building enough evidence to convict a suspect.

Dr Rose said work on a typical case began by carefully listening to the recorded evidence. This could include a series of telephone conversations about drug deals or shouting during a robbery.

He first decides whether the recording is of good enough quality to undergo further analysis. He will then often listen to the recorded evidence dozens of times to work out what is being said, familiarise himself with the voice, and pick out key words, phrases and sounds.

Dr Rose then uses acoustic computer technology to make a digital spectrogram of selected voice samples. These spectrograms reflect the average rate of vibration of the speaker's vocal cords. He uses these graphs to compare the recorded evidence and the suspect's voice sample.

There will always be differences between two given samples, but Dr Rose's task is to determine whether they are 'within speaker differences' (the same person talking) or 'between speaker differences' (different people talking).

He said that one challenge in comparing voice recordings was that people's speaking habits varied depending on the context. For instance, samples of a person shouting were very different from samples of the same person talking - they might not even sound like the same speaker.

"A common problem is that you speak very differently when you're being interviewed by the police in the station at three in the morning than when you're holding up a petrol station," Dr Rose said.

After his analysis, Dr Rose will give the jury a measure of the strength of the evidence using a likelihood ratio. He may tell the jury that, based on the evidence, you are twice as likely to observe the differences if they came from the same speaker than if they came from different speakers.

At the end of last year, Dr Rose was asked to give his expert opinion on a controversy in which the test cricketer, Shane Warne was alleged to have yelled "Can't bowl, can't throw" at a new team-member. Dr Rose said the voice sample was far too short to make any substantive claims about whether it was Warne speaking in the recording or not.

"To some people it may have sounded like Warne, but you have to be careful because there is this thing called the 'expectation effect'; if you think you're going to hear someone, you will tend to hear them," he said.